It has been known since the 1970s to form bearings based on polycrystalline ceramic. This is because, as regards bearings usually formed from monocrystalline ruby which is transparent, the studies of the 1970s, such as, for example, in the document GB 2 010 236, consisted in manufacturing bearings based on polycrystalline ceramic which are capable of replacing, at reduced cost, the monocrystalline ruby, that is to say while coming as close as possible thereto with regard to the color and the transparency.
The document GB 2 010 236 thus provides for the formation of a polycrystalline ceramic based on alumina, on chromium oxide and on magnesium oxide and for the addition thereto of strontium oxide in order to obtain a polycrystalline ceramic similar to a monocrystalline ruby both in terms of color and of transparency.
However, while this composition may be satisfactory as a replacement for the monocrystalline ruby, the known polycrystalline ceramics are not, on the other hand, satisfactory in terms of appearance (excessively pink and excessively transparent color) and of mechanical characteristics (excessively low toughness) for them to be able to be used for a part of the exterior of a timepiece.